Greeting Swiss city at start of evening, be out for a long time (9)
I believe the answer is:
hibernate
'be out for a long time' is the definition.
(sleep or 'be out' through the winter)
'greeting swiss city at start of evening' is the wordplay.
'greeting' becomes 'hi' (hi is a kind of greeting).
'swiss city' becomes 'bern' (Bern is the capital of Switzerland).
'start of' indicates taking the first letters.
The first letter of 'evening' is 'e'.
'hi'+'bern'+'at'+'e'='HIBERNATE'
(Other definitions for hibernate that I've seen before include "Breathe in to settle down for winter sleep" , "go dormant" , "Of animals, sleep through the winter" , "Breathe in for long winter sleep" , "Pass the winter in a dormant state" .)